The Apaf-1*procaspase-9 apoptosome complex functions as a proteolytic-based molecular timer

EMBO J. 2009 Jul 8;28(13):1916-25. doi: 10.1038/emboj.2009.152. Epub 2009 Jun 4.

Abstract

During stress-induced apoptosis, the initiator caspase-9 is activated by the Apaf-1 apoptosome and must remain bound to retain significant catalytic activity. Nevertheless, in apoptotic cells the vast majority of processed caspase-9 is paradoxically observed outside the complex. We show herein that apoptosome-mediated cleavage of procaspase-9 occurs exclusively through a CARD-displacement mechanism, so that unlike the effector procaspase-3, procaspase-9 cannot be processed by the apoptosome as a typical substrate. Indeed, procaspase-9 possessed higher affinity for the apoptosome and could displace the processed caspase-9 from the complex, thereby facilitating a continuous cycle of procaspase-9 recruitment/activation, processing, and release from the complex. Owing to its rapid autocatalytic cleavage, however, procaspase-9 per se contributed little to the activation of procaspase-3. Thus, the Apaf-1 apoptosome functions as a proteolytic-based 'molecular timer', wherein the intracellular concentration of procaspase-9 sets the overall duration of the timer, procaspase-9 autoprocessing activates the timer, and the rate at which the processed caspase-9 dissociates from the complex (and thus loses its capacity to activate procaspase-3) dictates how fast the timer 'ticks' over.

Publication types

  • Research Support, N.I.H., Extramural
  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Apoptosis
  • Apoptosomes / genetics
  • Apoptosomes / isolation & purification
  • Apoptosomes / metabolism*
  • Apoptotic Protease-Activating Factor 1 / genetics
  • Apoptotic Protease-Activating Factor 1 / isolation & purification
  • Apoptotic Protease-Activating Factor 1 / metabolism*
  • Caspase 3 / genetics
  • Caspase 3 / isolation & purification
  • Caspase 3 / metabolism
  • Caspase 9 / genetics
  • Caspase 9 / isolation & purification
  • Caspase 9 / metabolism*
  • Cell Line
  • Cloning, Molecular
  • Enzyme Activation
  • Humans
  • Mice
  • Mutation

Substances

  • Apoptosomes
  • Apoptotic Protease-Activating Factor 1
  • Caspase 3
  • Caspase 9